1. Technical Field
Embodiments of the present invention relate to the field of drop-on-demand inkjet printers.
2. Description of the Related Arts
There are several methods in the art for propelling an ink droplet from a drop-on-demand inkjet printer. These methods include piezo-electric jets, electrostatic jets and thermal or bubble jets. In general, a printer has a print-head with multiple jets (channels). Most printers also pack the multiple channels close together to enhance printing speed and printing quality. However, this requirement leads to a problem known as crosstalk in many printers. Crosstalk is caused by a coupling of energy between firing channels. The energy is typically mechanical energy associated with the physical disturbance created to expel a drop or electrical energy associated with the electrical driving voltage. The effect of crosstalk is usually observed as a change in velocity and/or volume of an ejected drop of ink caused by the simultaneous firing (or prior) of one or more other channels. Crosstalk can result in degradation of print quality. There are usually several physical mechanisms by which mechanical energy is coupled from one channel into another. They could include paths through the common ink supply or paths through the common mechanical structure in the print-head. FIG. 1A illustrates a common mechanical structure of a length expander type of piezo-electric inkjet according to the prior art. As illustrated, a piezo-electric driver (e.g., transducer A 105, transducer B 110, transducer C 115, transducer D 120, transducer E 125, transducer F 130, and transducer G 135) exists for each separate transducer. Each of the transducers is in communication with the same mechanical transducer support structure 100. When a voltage is applied to a transducer or an existing voltage is rapidly changed, the transducer xe2x80x9cfiresxe2x80x9d (i.e., rapidly elongates), extending in a direction opposite the mechanical transducer support structure 100.
When fired, the transducer""s motion is coupled mechanically to all of the other transducers. This results in xe2x80x9cstructural crosstalk.xe2x80x9d In general, the crosstalk between any two channels results in changes in drop velocity and size, that can be positive or negative. However, for the length expander mechanism described above, the crosstalk between adjacent transducers is often negative. This can be seen by referring to FIG. 1B.
FIG. 1B illustrates a common mechanical structure of a length expander piezo-electric inkjet after a transducer is fired according to the prior art. The reason for negative crosstalk between adjacent transducers is illustrated by considering the common mechanical xe2x80x9crear mountxe2x80x9d (i.e., the mechanical transducer support structure 100) for the transducers as a beam. When one transducer is fired, it extends in length to push against an ink chamber, thus reducing the volume of the chamber in order to expel a drop of ink. This length extension also results in a reaction force in the opposite direction on the mounting beam. The beam is therefore pushed away from the ink chambers and thus the adjacent transducers are also pulled away from their ink chambers as shown in FIG. 1B.
As illustrated, when transducer D 120 is fired, it expands in length and its lower end is initially displaced in a downward direction to drive an ink drop out of the chamber. The other end, however, is displaced in the opposite direction, pushing against the mechanical transducer support structure 100, causing it to deform. This deformation is propagated as a mechanical wave in the mechanical transducer support structure 100 and the structure undergoes a damped vibration. The mechanical transducer support structure 100 typically deforms, as it is not possible to make it completely rigid. The adjacent transducers A 105, B 110, C 115, E 125, F 130, and G 135 are also pulled upward initially because they are also attached to the mechanical transducer support structure 100. If any of the adjacent transducers are fired at the same time as D 120, the initial upward motion will subtract from the firing motion, resulting in a smaller push on the chamber, resulting in a slower, smaller drop; thus, negative crosstalk. A similar explanation applies to the refill part of the drive pulse. Accordingly, after transducer D is fired, the mechanical wave propagates through the mechanical transducer support structure 100 for a period of time (e.g., time xe2x80x9cTxe2x80x9d) thereafter. Also, when fired, a transducer elongates and then undergoes an oscillatory expansion/contraction during the duration of the firing. These oscillations are also transmitted throughout the mechanical transducer support structure 100, and can result in structural crosstalk received by another transducer fired a short amount of time thereafter (or before), resulting in sub-optimal performance.
The mechanical wave coupling any transducer to another in the array of transducers travels at a speed determined by the geometry of the structure and the sound speed of the materials. The array of identical transducers can behave like an acoustic delay line and the mechanical wave propagation speed can be lower than the sound speed in the bulk materials. For coupling between transducers that are spaced more distantly, the mechanical disturbance may become weaker from attenuation and there may be a significant phase delay. Crosstalk between more distant transducers may therefore be weaker and, because of the phase delay, may be positive or negative. Also, crosstalk between nearby transducers firing at the same time may be changed from negative to positive and the magnitude of the crosstalk may be made stronger or weaker if a delay is introduced between the firings of the two transducers.
The above explanation of how crosstalk may vary in strength and from positive to negative has been illustrated by reference to a particular type of structural crosstalk in a length expander piezo-electric inkjet. However a similar variation of crosstalk with distance and/or firing phase delay can occur for other crosstalk mechanisms and other types of inkjet. In particular, for most types of crosstalk mechanism in any inkjet, it may be possible to change the sign and strength of the crosstalk by changing the firing phase delay.
Some current systems seek to minimize the effects of crosstalk by firing alternate channels after a selected delay time instead of firing all at the same time. This delay would result in an error in the location of the printed dot on the paper, but the error has been compensated for by off-setting the position of the jet orifices for the delayed channels. For example, if transducers C 115, D 120, and E 125 are all to be fired, rather than firing all at the same time, one can be fired before the others. Some systems fire the even transducers, then delay for a period of time, T/2, before firing the odd transducers. So the transducer D 120 would be fired, and after a (T/2) delay, transducers C 115 and E 125 would be fired.
The time, T, is the shortest time between possible firings of the same transducer. In this scheme, the delay period is chosen to be T/2 because that is the maximum period for separating the firings of adjacent channels. If T is large, then the delay time, T/2, may be sufficiently large that the crosstalk between adjacent channels may be small. However, a large value for T means that the maximum jet firing repetition rate would be low. Low firing repetition rates may not be desirable because the printing speed may be limited. Another disadvantage of the delay time T/2 is that this would result in a relatively large drop placement error so a corresponding orifice off-set correction may be needed.
The above scheme is referred to as a two phase delayed firing system. In other systems, the channels are grouped in threes with a delay of T/3 between adjacent channels in each group (3 phase delayed firing). According to such systems, transducer C 115 would be fired, then after a (T/3) delay, transducer D 120 would be fired, and then after another (T/3) delay (i.e., 2T/3 after firing transducer C 115), transducer E 125 would be fired. Four phase delayed firing systems have also been tried. The four phase firing system is designed according to the same principle as the two and three phase systems; that is the channels are divided into groups of four adjacent channels. Each channel in a group of four is fired at a time that is a multiple of T/4 different from its neighboring channel in the group.
However, such systems still typically experience much crosstalk because the timing of the delay is not optimized. In such systems, the delays are obtained by dividing the time, T, into n equal increments where n is the number of phases. The objective is to minimize crosstalk by separating the firing times of adjacent channels by as much as possible. However, in most cases, T is not sufficiently large and the mechanical wave is often still propagating throughout the print-head or ink passages even after a T/n delay, typically resulting in reduced but still unacceptably large crosstalk. Current systems are therefore deficient because they are not optimized to minimize crosstalk.
Other methods of reducing crosstalk include design changes made to the print-head. These methods are normally directed at just one mode of crosstalk and the design changes required often involve a compromise which may adversely affect other performance aspects of the print-head. The method of optimizing the delay between firing phases avoids these problems and can be used to reduce crosstalk on any print-head.
Another problem with prior methods is due to the length of the delay between the firings. Specifically, because the delay between firings can be long relative to the movement of the entire print-head, delaying a channel typically results in a displacement error of an ink droplet onto the paper. To minimize this error, current systems shift the orifice for an ink droplet to be produced in a horizontal direction away from the direction of movement of the print head. In other words, if the print-head in moving toward the right of a page, the orifice for a droplet to be produced is moved to the left. Because of print-head moves even though a droplet is delayed, the ink drop will be printed to the right of where it should be printed unless the orifice is moved to the left. However, moving such orifices adds an extra cost and layer of complexity to the manufacturing of the inkjet printing system.